We observed a linear (1-hit) (O - 6 Jm to the minus 2nd power)and dose-squared (2-hit) (0.5 - 6 Jm to the minus 2nd power) UV mutagenesis response in E. coli K-12 uvrB5, and a third mutation system that only occurs between approximately 1 -3 Jm to the minus 2nd power (?-hit). We will now determine if back mutations and suppressor mutations occur by 1-hit, 2-hit or ?-hit kinetics. We have constructed a new series of multiply-mutant strains, and will determine the effect of recF, recB, lexA, and uvrD mutations on radiation sensitivity, genetic recombination and inducible repair in the uvrB background.